In paper slitting, we use ‘fuzzy’ wound roll edges (something easy to see) as a proxy for dust (the problem of concern). The black rag test (actually a specific 3M scrubby pad) is just one way to make dust less subjective by comparing it to several graded photos. In both cases the proxy is visual. In the rough roll edge, it is a particular form that looks shattered but is called 'fuzzy.' In the black rag it is the greyness of the rag. In the advanced winding class, we get into more complicated proxies. One such is how to ‘see’ gage variation on clear film at a glance by relative transparency and translucency. Several physical principles are strung together to do this. The high gage band has higher interlayer pressure there that pushes air into neighboring lanes which then makes them more translucent due to diffraction of light at the air-film interface.
Come to my winding school and I will show how these relationships are strung together for practical problem solving. As a bonus I will show you how to calculate the air entrained, measure the air entrained by weighing a roll and why air and tightness are a monotonic relationship. But really, what I want most to teach is how to tell at a glance gage variation coming from upstream manufacturing or converting.